1. Field of the Invention
The present invention relates to a sheet processing device that performs processing such as the sorting and binding of sheets and, more particularly, to sheet conveying sections for conveying sheets to a sheet processing section. Still more particularly, the present invention relates to a sheet processing device capable of keeping large amounts of sheets on standby, irrespective of the size and material of the sheets and without the need to upsize the device itself.
2. Description of the Related Art
In recent years, some image forming apparatuses such as copy machines, printers, facsimiles, and the like, have a sheet processing device that selectively applies processing such as stapling or saddle stitch bookbinding to sheets having an image formed thereon.
In such processing devices, a buffer is provided upstream of the processing section, for keeping a plurality of sheets on standby, whereby processing can be performed without reducing productivity of the image forming apparatus.
FIG. 10 shows an image forming apparatus capable of high-speed, large-capacity image formation and having a conventional sheet processing device. Sheets S discharged from an image forming apparatus 100 are conveyed to a sheet processing device 102, and then aligned by a processing tray 103 provided in the sheet processing device 102. Thereafter, the sheets are stapled by a stapler 104, and discharged to a stack tray 105.
While the sheet bundle is being stapled in this manner, subsequent sheets cannot be delivered to the processing tray 103. In such a case, therefore, after a conveying path has been switched by a flapper 102a, the sheets are stacked one on top of another, with the front ends thereof aligned, and they are wound around a buffer roller 106. In this state, the sheets are kept on standby in buffer path Pa.
After the above-described sheets having been stapled are discharged to the stack tray 105, the sheets in buffer path Pa are conveyed to the processing tray section 103 in a stacked state. By repeating this operation, the stapling treatment can be performed without reduction in productivity of the image forming apparatus 100.
In FIG. 10, reference numeral 114 denotes an inserter for setting sheets that are not to be printed on, references 107a and 107b are accommodating sections provided in the inserter 114, and reference numeral 115 denotes a feed section for feeding sheets S accommodated in the accommodating sections 107a and 107b. 
Here, the inserter 114 is for accommodating sheets S1 to be inserted in the accommodating sections 107a and 107b, and conveying them to the sheet processing device 102, while merging therewith the sheets S from the image forming section 100A. For example, the merging occurs when the present image forming apparatus 100 is dedicated to a black-and-white printer, and color print sheets are to be inserted into a bound book, or when sheets to be inserted are specific sheets (e.g., heat-sensitive sheets) that cannot be passed through the image forming section 100A of the present image forming apparatus 100.
In FIG. 10, reference numeral 108 denotes a saddle stitching section that performs processing from stapling to bundle folding on-line. The sheet processing device 102 connected to the above-described high-speed, large-capacity image forming apparatus 100 includes, besides the saddle stitching section 108, a “perfect binding” bookbinder that binds a book by pasting a spine to an aligned sheet bundle, and an offset stacker that performs only sorting and alignment on-line, with the bookbinding operation being conducted off-line. These may be connected to the system for use, depending on the purpose for using.
However, such a conventional sheet processing device has involved a problem in that the device itself increases in size because it has therein a buffer path Pa.
Specifically, under the size condition of the buffer roller 106 shown in FIG. 10, large-sized sheets such as A3 or B4 cannot be wound around the buffer roller 106. When attempting to keep large-sized sheets on standby, a larger buffer roller is required.
Also, when attempting to wind thick sheets around the buffer roller 106, high stiffness of the thick sheets increases the conveyance resistance, thereby causing skewing or jamming of the sheets. Furthermore, when the buffer path Pa is formed by the buffer roller 106, there is a limit to the number of sheets to be stacked, because sheets are stacked while being conveyed in the buffer path Pa with a curvature. This might reduce the productivity of the image forming apparatus when performing time-consuming processing such as “perfect binding” book-binding.